1. Field of the Invention
The present invention relates to balanced positioning systems. More particularly, the invention relates to such systems in lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
2. Description of the Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatus are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions which are successively irradiated via the mask, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In a lithographic apparatus, reactions on the machine frame to acceleration forces used to position the mask (reticle) and substrate (wafer) to nanometer accuracies are a major cause of vibration, impairing the accuracy of the apparatus. To minimise the effects of vibrations, it is possible to provide an isolated metrology frame on which all position sensing devices are mounted, and to channel all reaction forces to a so-called force or reaction frame that is separated from the remainder of the apparatus.
U.S. Pat. No. 5,208,497 describes a system in which the reaction of the driving force is channeled to a balance mass which is normally heavier than the driven mass and which is free to move relative to the remainder of the apparatus. The reaction force is spent in accelerating the balance mass and does not significantly affect the remainder of the apparatus. However, the concept disclosed in U.S. Pat. No. 5,208,497 is only effective for reaction forces in one direction and is not readily extendable to systems having multiple degrees of freedom. Balance masses moveable in three degrees of freedom in a plane are described in WO 98/40791 and WO 98/28665 (mentioned above).
EP-A-0,557,100 describes a system which relies on actively driving two masses in opposite directions so that the reaction forces are equal and opposite and so cancel out. The system described operates in two dimensions but the active positioning of the balance mass necessitates a second positioning system of equal quality and capability to that driving the primary object.
None of the above systems is particularly effective at counteracting yawing moments which may be induced by adjustments of the rotational position of the driven mass or because of misalignment between the line of action of forces exerted on the driven body and its center of mass.
U.S. Pat. No. 5, 815,246 discloses a positioning system having a first balance mass free to move in an XY plane, i.e. to translate in X and Y and rotate about axes parallel to the Z direction. To control rotation of the first balance mass, a fly wheel, forming a second balance mass, is driven by a rotation motor mounted on the first balance mass to exert a counter-acting torque. Controlling rotation of the first balance mass therefore requires accurate control of the rotation and the flywheel. Any delay in this control or imbalance of the flywheel will cause vibration.
An object of the present invention is to provide an improved balanced positioning system for counteracting yawing moments in the driven mass and preferably also force balancing in at least two translational degrees of freedom.
According to the present invention there is provided a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate;
a projection system for imaging irradiated portions of the mask onto target portions of the substrate; characterized by:
a balanced positioning system for positioning at least one of said object tables and comprising:
first and second balance masses;
bearing means for supporting said first and second balance masses so as to be substantially free to translate in at least one direction; and
driving means for acting directly between said one object table and said first and second balance masses to rotate said object table about an axis perpendicular to said one direction, said driving means being arranged to exert linear forces on said first and second balance masses in opposite directions to effect said rotation of said object table.
By providing two balance masses that can translate in at least one direction, the torque required to drive the object table to adjust its rotational position, or to compensate for torques induced by other driving forces can be provided as the sum of two linearly acting forces acting between the object table and the two balance masses. The reaction forces on the two balance masses will cause them to move linearly, which can easily be accommodated. In other words, the reaction to a torque exerted on the driven object table is converted to translations of the two balance masses and no rotational movement of the balance mass occurs. It will be appreciated that if a rotational motion of the object table is combined with a linear motion, the net forces acting on each balance mass may be in the same direction, though different in magnitude.
According to a yet further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging irradiated portions of the mask onto target portions of the substrate; the method comprising the steps of:
providing a mask bearing a pattern to said first object table;
providing a substrate provided with a radiation-sensitive layer to said second object table;
irradiating portions of the mask and imaging said irradiated portions of the mask onto said target portions of said substrate; characterized in that:
at least one of said object tables is positioned using a positioning system which includes first and second balance masses free to move in at least one direction and drive means acting between said one object table and said balance masses; and
during or prior to said irradiating step said one object table is rotated by exerting oppositely directed forces between it and said first and second balance masses.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet radiation (e.g. at a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), EUV, X-rays, electrons and ions.
Embodiments of the present invention are described below making reference to a Cartesian coordinate system with axes denoted X, Y and Z in which the XY plane is parallel to the nominal substrate and reticle surfaces. The notation Ri is used to denote rotation about an axis parallel to the I direction.